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HF transmitter causing TV to shut off

Jul 24th 2011, 10:34

w1rfi

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Joined: Apr 4th 1998, 00:00
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An ARRL member inquired:

Hello. I am having trouble when I use my radio it shuts off the TV. I have a low band filter and hooked it up between the coax and the watt meter and I am still having trouble. Maybe I hooked it up the wrong way. I am putting out 300 watts from my AMP. How can I prevent this from happening.

Thank you for any advice you can give me.
Jul 25th 2011, 00:02

w1rfi

Super Moderator

Joined: Apr 4th 1998, 00:00
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In the case of your transmitter shutting off the TV, your transmitted signal is getting into the TV power supply, control circuitry or even the high-voltage circuit and causing the TV to shut down.

In this case, the problem is almost certainly caused by your fundamental signal -- the strongest signal from your station and the one you are trying to transmit. For that reason, a typical transmitter TVI filter will generally have no effect. Most transmitter filters are low-pass filters designed to pass your fundamental signal, but attenuate harmonics and other spurious signals that appear above 30 MHz.

The problem must be cured at the TV.

To start troubleshooting the problem, we need to find out how the signal is getting into the TV. While it is possible that internal wiring of the TV may pick up your signal (this is called direct pickup), more than likely, it is the coaxial feed line or electrical system that is the ingress mechansm.

First, simplify this problem. If your TV is in a complex installation involving DVDs or other video devices, computer systems, home theatre, etc., temporarily simplify the problem to a system that includes only the TV and coaxial feed line. See if the problem still exists. If it does not, start adding things back one at a time, to get some idea of what other equipment may be involved.

With a configuration that causes the problem, first disconnect the coaxial cable from the TV. See if this eliminates the problem. If it does, this is almost certainly a common-mode problem, meaning that your signal is being picked up on the coaxial cable and conducted down the shield and inside wire in phase to the affected TV.

You can cure this with a common-mode choke. Obtain an FT-240-31 ferrite core (try Amidon, Dan's Small Parts and Palomar) and wrap about 10 turns of the coax onto it, just before it connects to the TV. Sometimes, there is not enough coax to accomplish this, so you want to get a piece of high-grade coax and a barrel connector and add the filter to your existing wire.

If the problem does not go away when you disconnect the coaxial cable, try a similar choke on the ac mains. You may also need to use a brute-force ac-line filter on the ac mains. Some surge supressors are also EMI filters. I will get our RFI engineer, Mike Gruber, W1MG, to do a followup post on Monday with a list of filter suppliers.

If you can filter the ac mains and take care of the shutoff problem, next reconnect the coaxial cable and verify that the problem is still corrected. Sometimes, RFI problems have multiple causes.

You may want to look closely at your station, as well. If your antenna is close to the TV and its wiring, you may be able to move the antenna. If the TV is on the same circuit as your transmitter, you may want to try to correct that, although rewiring a house to solve an RFI problem is not always a practical option. If your station grounding is very close to the TV and its wiring, that could also be part of the problem, althoughth this is unlikely.]

If you do all this and you still have a problem, it is probably internal to the TV. The manufacturer may have identified a cure, although this is unlikely.

One last point -- many hams read ARRL's advice and see the word "ferrite" and think that all ferrites are the same. Ferrites like clamp-on beads are much more convenient than obtaining a ferrite toroidal core and wrapping 10 turns of coax onto it, so hams get the beads and put one or two at various points. Unfortunately, these beads do not work as well as a multi-turn common-mode choke wound on a ferrite core with the correct properties. It's okay to try unknown ferrites, and if they work, you can use them, but if something other than an FT-240-31 core doesn't work, obtain the correct core and install it as described.

Good luck. If these things don't work, contact the RFI desk at ARRL HQ and Mike Gruber or I can talk this out with you.

73,
Ed Hare, W1RFI
ARRL Lab
Technical forums moderator
Aug 16th 2011, 16:32

N5LRZ

Joined: Apr 4th 1998, 00:00
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As with all things in consumer electronics these days, there is little or in most cases absolutely no filtering for strong external Radio Frequency. As a result many problems, one of which you mentioned, can happen.

The cure, not that it will do any good: call the company that built the Television and ask them for a filter kit/not that they will have one to begin with. Almost all th time there will be no cure other than just turning off th TV
Oct 27th 2011, 20:04

NA9DS

Joined: Nov 19th 2001, 00:00
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Total Posts: 0
I had a similar problem with and LED TV, but mine would turn on and refuse to turn off. I followed the basic advise above by disconnecting everything and found the signal was getting in primarily by the DVD player. I put some common mode chokes on the cable and AC, which helped, but when I disconnected the DVD player, the problem went away totally. I just leave it disconnected and will plug it back in if I need it. I may try a choke on that also if it gets annoying.
Hope this helps someone.

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